Conventional electromechanical cables for oil well logging include insulated metal conductors for the transmission of electrical signals. Such cables have signal transmission bandwidths that are limited to about 100 KHz over lengths that correspond to typical depths of oil wells, 12,000 to 20,000 feet. Much of the information that is obtainable with modern logging tools is not retrievable from down the well bore due to the restricted signal bandwidth that is characteristic of state-of-the-art conventional logging cables. Consequently, a need exists to provide oil well logging cables that have substantially higher signal transmission bandwidths. Optical fibers can provide signal transmission bandwidths one to three orders of magnitude higher than the insulated wires that are used in conventional well logging cables.
Glass optical fibers have two properties which make it difficult to successfully incorporate them into strain cables. These properties are static fatigue degradation and microbending loss.
Silica glass fibers have small cracks (microcracks) on their surface. The depth of these microcracks can increase through a stress-accelerated chemical reaction between the silica glass and moisture, called static fatigue. The tensile strength of the glass fiber decreases substantially as the microcracks increase in depth. Glass is an elastic material with a high Young's modulus. Strain in a glass optical fiber generates tensile stress and results in static fatigue. Thus, glass optical fibers are not suitable for use under high strain (&gt;0.5%) in the presence of moisture over extended periods of time.
Optical fibers transmit light signals by the principle of total internal reflection. This principal depends upon the light rays being totally reflected back into the core region each time they impinge upon the core to cladding interface of the optical fiber. Total internal reflection can only occur when the angle of incidence between the rays and the core to cladding interface is below a certain critical value. Bending of an optical fiber causes some of the light which is propagating in the fiber core to impinge upon the core to cladding interface at angles of incidence greater than the minimum value and to be refracted out of the optical core and lost. The amount of the light that is lost becomes greater as the effective diameter of the bend becomes smaller. When the bending of the optical fiber is caused by deflection due to local lateral forces, the resulting decrease in signal strength is called microbending loss. When an optical fiber is deflected by a local inhomogeneity, such as a lump in its coating layers, the effective diameter of the bend depends upon the local strain the fiber is under. Generally, the fiber will bend to a smaller effective diameter as the strain level it is under increases. Consequently, higher strain levels result in higher levels of microbending loss.
A necessary condition for accurate logging of a well bore is an accurate knowledge of the position of the logging tool within the well bore. The position of the tool is defined by the actual length of logging cable that is suspended in the well bore. The actual length of suspended cable can be determined from a knowledge of the amount of unstressed cable length that has been lowered into the well plus a knowledge of the elongation versus tension characteristics of the cable and the tension along the suspended cable length. The amount of unstressed cable length that has been lowered into the well bore can be precisely measured. The tension profile along the suspended length of cable can be accurately calculated. Therefore, the actual length of cable suspended in the well can be accurately determined if the elongation versus tension characteristics of the cable are accurately known and are repeatable.
Conventional electromechanical cables for well logging can be constructed to withstand harsh high temperature environments and to accept high levels of axial strain while still remaining functional. More specifically, for example, each conductor element in a conventional logging cable comprises a bundle of copper wires. The copper wires yield inelastically at low strain. When the cable is alternately stretched and relaxed, the copper does not fully return to its original state and eventually the copper wires become brittle, due to strain hardening, and break. However, even this serious condition does not necessarily render the cable inoperable because a break in one or more wires with adjacent nonbroken wires permits the current to be passed to the neighboring wires and thus the conductor still appears whole and the cable remains functional. Thus, conventional logging cables can withstand considerable inelastic and elastic strain and still remain functional.
Well logging cables are generally constructed with two layers of external steel armor wires. The armor wires are preformed and applied in helices of opposing handedness to prevent the cable from unwinding when supporting a free hanging load. Inside the armored jacket can be seven insulated copper conductors laid six around one in helices generally of opposite handedness to those of the steel wires in the inner armor layer. However, there is no definite relationship between the helices of the copper conductors and those of the inner armor wires since they are added in separate fabrication steps and usually with a bedding layer of a pliant material therebetween. A result of this conventional cabling geometry is that the interface between the inner armor wires and the underlying insulated conductors consists of a multiplicity of cross-over points separated by the pliant bedding material.
When a conventional well logging cable is tensioned at elevated temperatures, it will elongate by an amount which is not acurately predictable. This is because the elongation consists of two parts, one that is linear and one that is highly nonlinear and inelastic. The inelastic part occurs because the armor wires inelastically deform the underlying compliant bedding and the wire insulation, due to very high local stresses at the crossover points, and take on a smaller pitch diameter. The inelastic part of the cable elongation is not very predictable or repeatable and consequently the position of the logging probe will not be accurately known.
In order to prevent inelastic strain from occurring in use, conventional logging cables are given a hot prestretch during fabrication. When properly conducted, the hot prestretch operation will result in a cable that exhibits a linear and elastic elongation in response to tension. The hot prestretching operation imparts a permanent (inelastic) strain of between 3/4 to 11/2 percent to conventional seven-conductor logging cables. Hot prestretching of a conventionally designed armored cable containing one or more optical fibers within its core would leave the glass optical fibers under a permanent elongation of 3/4 to 11/2 percent. Optical fibers in cables subjected to these high permanent strain levels would soon fail from static fatigue and/or exhibit intolerably high microbending losses. It is apparent that conventional prestretching technology cannot be applied to armored optical fiber cables. Thus, it would be highly desirable to have an armored fiber optic cable which overcomes these and other difficulties and permits the expansion of optical fiber communications technology into areas of harsh environments.